1. Field of the Invention
This invention relates generally to semiconductor fabrication, and more particularly to an apparatus for implanting ions that incorporates multiple vacuum chambers.
2. Description of the Related Art
Ion implantation has been used for many years as a means of adding impurity ions into the surfaces of semiconductor wafers and various circuit structures implemented thereon.
In general, ion implantation involves the bombardment of a targeted surface with energetic charged atoms or molecules. During implantation, secondary electrons and other charged species are ejected from the targeted surface. This exodus of negatively charged particles from the targeted surface can lead to a positive charge buildup, particularly where the targeted surface is an insulating film or a polycrystalline silicon film positioned on an underlying oxide film. The positive charge buildup can alter the charge balance in the ion beam and lead to significant dose variations across the substrate. In some circumstances, the charge buildup may damage the surface of the substrate by inducing microscopic craters via oxide breakdown and degrade the transconductance and subthreshold characteristics of the semiconductor device.
One conventional method for addressing the problem of positive charge buildup during implantation involves the use of an electron flood gun. The electron flood gun directs a stream of low energy electrons at the substrate during implantation. The purpose of the low energy electron stream is to neutralize positive charge buildup. In order to attract the flood gun electron beam to the substrate surface, the substrate must be biased relative to the flood gun beam, normally at about 3 to 4 volts for various types of implanters. Depending upon the geometry of the implanted structures, this bias can lead to the trapping of both positive and negative charges, at locally high levels, particularly near edge structures, such as the edges of a gate electrode and the edges of an underlying gate oxide film.
Typical conventional implanters include an ion source and an ion accelerator and a wafer chamber. The wafer chamber and the enclosures for the accelerator and ion source are pumped down to a relatively high vacuum. Pressure levels of 10.sup.-6 torr or lower are typical. The high vacuum is used to minimize the formation of neutrals by collision of beam ions with residual gas atoms. Thus, the conventional implanter contemplates substantial evacuation and thus very low pressures in the wafer chamber and thus very low concentrations of atomic species that might otherwise be utilized to interact with the wafer surface and neutralize charge buildup.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.